The present invention relates to an image-forming material comprising a thermal transfer sheet and an image-receiving sheet, and specifically an image-forming material and image formation process that provide high-resolution full color images with use of laser light. More specifically, the invention relates to an image-forming material and image formation process useful for the production of color proofs (DDCP, direct digital color proof) in the graphic arts field or mask images by laser recording based on digital image signals.
In the graphic arts industry, a set of printing plates are prepared with use of a set of color separation film images made with lith films from a color original. In general, prior to the actual press operation, in order to check possible errors in color separation procedures and the necessity of color correction, color proofs are made from the color separation film images. Color proofing materials are expected to have a high resolution needed for faithful reproduction of halftone images and an out standing process stability. Further, to obtain color proofs that well simulate actual printed matters, they should preferably be made of the actual printing stock as the substrate, and pigments as the coloring agents both used for the actual press operation. Still further, color proofs should preferably be produced by a dry process with out using a developer liquid.
Various image-recording systems are being developed that can produce color proofs via a dry process directly from digital signals along with the prevalence of electronic systems in pre-press works in these days. Such electronic systems aim at the production of high-quality color proofs generally reproducing halftone images of 150 lines per inch or higher. To reproduce high-quality proof images from digital signals, a laser light is used as recording energy because of its capability of modulation by digital signals and realization of an extremely fine beam. Accordingly, the development of recording materials that exhibit a sufficiently high recording sensitivity to a laser light and a high resolution enabling a precise reproduction of minute halftone dots are demanded.
As the recording material used for image transfer processes based on a laser light, a thermally meltable transfer sheet is set forth in Japanese Patent Laid-Open No. 58045/1993 which comprises a substrate, a light-to-heat conversion layer generating heat by laser light absorption and an image-forming layer containing a pigment dispersed in a thermally meltable wax or binder, both layers being provided on the substrate in this order. In the image-forming process using such a recording material, the heat generated at the laser-irradiated area of the light-to-heat conversion layer melts the image-forming layer at the corresponding area, and the melted layer is transferred onto an image-receiving sheet superimposed on the transfer sheet to form a transferred image.
Japanese Patent Laid-Open No. 219052/1994 discloses a thermal transfer sheet comprising a light-to-heat conversion layer containing a light-to-heat conversion material, a very thin (0.03 to 0.3 xcexcm thick) thermal stripping layer, and an image-forming layer containing a colorant, all provided on a substrate in this order. When a laser light is irradiated on this type of thermal transfer sheet, the bonding force between the image-forming layer and the light-to-heat conversion layer secured by the presence of the thermal stripping layer is reduced, and thus a high-quality image is formed on an image-receiving sheet superimposed on the thermal transfer sheet. This image formation process makes use of the so-called xe2x80x98abrasionxe2x80x99 phenomenon; i.e., at the area where the laser light was irradiated, the thermal stripping layer is partly vaporized by decomposition to weaken the bonding between the image-forming layer and the light-to-heat conversion layer there, thus causing the image-forming layer at the irradiated area to be transferred onto the image-receiving sheet superimposed on the thermal transfer sheet.
These image formation processes have a number of advantages including the capability of using an actual printing stock coated with an image-receiving (adhesive) layer as the image-receiving sheet material and the ease with which it can produce multi-color images by simply sequentially transferring differently colored images on a single image-receiving sheet. In particular, the abrasion-based image formation process, which has a prominent feature of readily producing high-quality images, is useful for the preparation of color proofs (DDCP: Direct Digital Color Proofs) and high-resolution mask images.
When a thermal transfer sheet for use in color image formation gives image defects, the commercial value thereof is noticeably damaged. One reason of image defect generation is the damage of the image-forming layer causing a partial lack of the layer. At such lacking areas, no image transfer takes place thus giving rise to a void in an image. The thermal transfer sheet undergoes such damages during manufacture, processing or image recording due to the rubbing of the front surface of the sheet against the rear surface thereof, for example, in an image-recording apparatus. Especially when the area of the image is large, the probability of image defect generation rises in proportion to the image area. Accordingly the more scarce defect generation is demanded for thermal transfer sheets used for the production of the larger images.
To prevent the generation of such image defects, Japanese Patent Laid-Open No. 270154/1993 discloses a method of using a specified polyester and an acrylate-styrene copolymer as the binder of the image-forming layer. Alternatively, prevention of image defect generation by providing a protective coating on the image-forming layer is also in practical use.
Although one can decrease the frequency of defect generation to some extent with these countermeasures, images of large areas are still accompanied by a practical trouble since the number of image defects is proportional to the image area. Moreover, introduction of a protective layer that can suppress the generation of image defects suffers from the drawback that a large amount of recording energy is required for thermal image transfer.
As another problem, a light-to-heat conversion layer comprising carbon black used as the light-to-heat conversion material, which is preferred as regard to the material cost and the absorption efficiency for laser lights, has a drawback that the image-forming layer provided on the light-to-heat conversion layer is susceptible to mechanical damaging because of the insufficient cohesive energy of the light-to-heat conversion layer. Though the mechanical damages caused by the use of such carbon black-based light-to-heat conversion layer may be prevented by raising the scratch resistance of the image-forming layer, there arises another problem of an insufficient reflection optical density of the transferred image.
Still another type of trouble exists as for image defects. Recently, multi-beams of a laser light are used for laser image recording for the purpose of curtailing the recording time. When a conventional thermal transfer sheet is exposed to a multi-beam laser light for image recording, various troubles associated with image defects tend to occur such as transfer of the light-to-heat conversion layer onto the image-receiving sheet or transfer of the image-forming layer at non-irradiated areas instead of transfer at irradiated areas (reversal mode transfer). Furthermore, the thermal transfer sheets must be handled with a great care so as to cause no peeling-off or damaging of the image-forming layer thereof, which has demanded a high skill for the operator.
As has been pointed out previously, the image-forming material of the invention is expected to be provided with a high process stability. For example, the material must exhibit desirable conveyance and stacking properties since multiple image-receiving sheets must be stacked up after recording.
To cope with the situation described hereinabove, the object of the invention is to provide an image-forming material comprising a thermal transfer sheet that can prevent the generation of image defects even when the image area is large or when carbon black is used in the light-to-heat conversion layer and that can form thermally transferred images of a sufficiently high optical reflection density.
Another object of the invention is to provide an image-forming material comprising a thermal transfer sheet that undergoes a desirable image transfer only laser-irradiated areas when a multi-beam laser light with high energy density is used to provide images on an image-receiving sheet, and that is provided with an improved handling property.
Still another object of the invention is to provide an image-forming material comprising an image-receiving sheet used for an image formation process based on abrasion, the image-receiving sheet excelling in conveyance and stacking properties and capable of readily forming high-resolution images suited for color proofs and precise masks with a high process stability.
The invention also aims to provide an image formation process using such improved thermal transfer sheets and image-receiving sheets as described hereinabove.
The above-cited objects can be achieved by the materials and process described in the following.
(1) An image-forming material comprising an image-receiving sheet having at least an image-receiving layer on a substrate and plural differently colored thermal transfer sheets each having at least a light-to-heat conversion layer and an image-forming layer on a substrate, the outermost surface of the side of said thermal transfer sheet in which said image-forming layer is provided exhibiting a scratch resistance of 50 to 200 g when said surface is scratched with a stylus with a radius of curvature of 0.25 mm at a velocity of 1 cm/sec.
(2) An image-forming material set forth in (1) in which said scratch resistance is 100 to 200 g.
(3) An image-forming material set forth in (1) or (2) in which the area of an image formed in said image-forming layer is at least 1000 cm2.
(4) An image-forming material set forth in (1) to (3) in which said thermal transfer sheet is used to form a color image on the image-receiving sheet.
(5) An image-forming material set forth in (1) to (4) in which the outermost surface of the side on which said image-forming layer is provided is that of said image-forming layer itself.
(6) An image-forming material set forth in (1) to (5) in which said light-to-heat conversion layer contains carbon black.
(7) An image-forming material set forth in (1) that exhibits a peeling force of 5.9 N/m to 0.5 N/m when the light-to-heat conversion layer of said thermal transfer sheet and the image-forming layer are peeled off from the substrate of the thermal transfer sheet with a peeling angle of 90 deg and at a peeling velocity of 500 mm/min after the image-receiving layer of said image-receiving sheet is brought into face-to-face contact with the image-forming layer of said thermal transfer sheet, and then the laminated body is subjected to laser light irradiation from the substrate side of the thermal transfer sheet.
(8) An image-forming material set forth in (7) that exhibits a peeling force of at least 0.98 N/m when the light-to-heat conversion layer and the image-forming layer of said thermal transfer sheet are peeled off from the substrate of said thermal transfer sheet with a peeling angle of 90 deg at a peeling velocity of 500 mm/min before the thermal transfer sheet is brought into contact with the image-receiving layer of said image-receiving sheet.
(9) An image-forming material set forth in (8) in which the peeling force described in (8) is larger than the value set forth in (7).
(10) An image-forming material set forth in (1) in which the dynamic frictional force of the surface (image-receiving surface) having the image-receiving layer of said image-receiving sheet and that opposite to said surface (back surface) do not exceed 40 gf, and in which the surface roughness Rz of said image-receiving surface does not exceed 4 xcexcm, and in which the surface roughness Rz of said back surface does not exceed 8 xcexcm.
(11) An image-forming material set forth in (10) in which the surface roughness Rz of said image-receiving surface does not exceed 3 xcexcm, and in which the surface roughness Rz of said back surface does not exceed 5 xcexcm.
(12) An image-forming method using an image-forming material comprising an image-receiving sheet having at least an image-receiving layer on a substrate and four or more kinds of yellow, magenta, cyan and black thermal transfer sheets comprising, the method comprising superimposing each of said thermal transfer sheets on said image-receiving sheet in such a manner that the image-forming layer of said image-forming sheet and the image-receiving layer of said image-receiving sheet is in direct contact, exposing the superimposed laminate to a laser light to transfer the laser-irradiated region of said image-forming layer onto the image-receiving layer of said image-receiving sheet, thus forming an image wherein said image-forming material is selected from those set forth in one of (1) to (11).
(13) A method of forming a color proof in which a multi-color image formed on the image-receiving layer by the method set forth in (12) is again transferred together with the image-receiving layer onto an actual printing stock.